Formation of the rotor track of a rotary engine

ABSTRACT

This invention relates to a method of electroforming the epitrochoidal track for the rotor of the Wankel internal combustion engine. The method involves the deposition of a sequence of plates on a mandrel of epitrochoidal shape and casting the aluminum alloy housing onto the final plate. The sequence of plates essentially involves, (1) a first plate with poor adhesion to the mandrel, (2) a wear-resistant plate and (3) a final plate to aid in securing good adhesion of the aluminum alloy housing cast thereon. These are the main steps. Intermediate steps may also be involved such as using two thin plates in step (1) to obtain poor adhesion.

United States Patent Brown FORMATION OF THE ROTOR TRACK OF A ROTARY ENGINE Inventor: Henry Brown, Huntington Woods,

Mich.

Assignee: Oxy Metal Finishing Corporation,

Warren, Mich.

Filed: Dec. 18, 1972 Appl. No.: 316,207

US. Cl 204/9, 29/5273, 164/131, 204/4, 418/178 Int. Cl. C23b 7/02, C23b 7/00 Field of Search 204/9, 3, 4; 418/178; 164/46, 98, 75, 100, 131, 132; 29/5273; 123/8.0l

References Cited UNITED STATES PATENTS 11/1964 Bentele 418/178 Primary ExaminerT. M. Tufarielllo Attorney, Agent, or FirmB. F. Claeboe [57] ABSTRACT This invention relates to a method of electroforming the epitrochoidal track for the rotor of the Wankel in ternal combustion engine. The method involves the deposition of a sequence of plates on a mandrel of epitrochoidal shape and casting the aluminum alloy housing onto the final plate. The sequence of plates essentially involves, (1) a first plate with poor adhesion to the mandrel, (2) a wear-resistant plate and (3) a final plate to aid in securing good adhesion of the aluminum alloy housing cast thereon. These are the main steps. Intermediate steps may also be involved such as using two thin plates in step (1) to obtain poor adhesion.

9 Claims, No Drawings FORMATION OF THE ROTOR TRACK OF A ROTARY ENGINE This invention relates to both a method and certain sequences of electroplating steps for electroforming a wear-resistant track in the aluminum housing of rotary combustion engines, especially the electroforming of the epitrochoidal track of the aluminum housing of the Wankel rotary gasoline engine. In particular, the method relates to electroforming the track by electroplating a series of plates on a mandrel that has for the Wankel rotary engine, the epitrochoidal shape and size of the track, and then die casting the aluminum housing on to the last electroplate and removing the housing with the attached electroplates containing the wear plate or plates.

The present method used for obtaining a wear resistant surface for the epitrochoidal track in the aluminum housing of the Wankel rotary engine is to die cast the aluminum housing directly on to the mandrel or on to a porous sprayed iron transplant applied to the mandrel, and then electroplating a thick nickel plate containing codeposited fine silicon carbide particles on to the epitrochoidal track on the inside of the aluminum housing or a thick chromium plate on top of the iron transplant. Then the thick plate is machined to exact size. These methods require internal anodes to plate the track, and much metal must be wasted because an extra thick plate must be electrodeposited, usually twice the final thickness to be left after machining or grinding and honing to the final size for the bore. Also the time needed to plate the thick plate adds greatly to the expense.

In the method of this invention it is the mandrel that is plated instead of the internal track of the aluminum housing, and using a mandrel with a high finish and exact size, very little if any machining is needed for the epitrochoidal surface. Thus thinner hard plate can be used and very little if any plated metal is lost by machining.

To make possible successful commercial use of the system wherein the outside of the mandrel is electroplated instead of the inside of the aluminum alloy housing, it is necessary l that the plate can be readily detached from the mandrel and (2) that the aluminum die cast housing adheres with good bond to the composite plates. These two requirements are more important than the goal to eliminate any machining of the plate after its detachment from the mandrel which would require starting with a highly finished very accurate mandrel. In any case, using a highly finished accurate mandrel, much less machining would be needed for a final accurate surface with this method than for the present method of plating the inside of the aluminum alloy housing. The present method requires the electrodeposition of very thick plate, around mils, to be sure to have sufficient plate left (4 to 6 mils) after machining, and this is a great economic loss. This expensive procedure which is used to plate the inside of the aluminum housing results from the fact that the plate becomes less smooth as the thickness is increased and also because the plate distribution is relatively poor from the baths used (poor throwing power of the acidic baths used) which results in a non-uniform thickness of plate on the epitrochoidal surface. In the method of plating the outside of the mandrel, it is the first part of the plate which is used and it is as smooth as the mandrel and as accurate in dimensions as the mandrel. Thus, even if the mandrel is not absolutely accurate in dimensions, the amount of machining needed is far less than the present method of plating the inside of the housing. Furthermore, the first plate or plates deposited on the mandrel may be relatively thin plates of less hard material than the wear-resistant plates, and these plates can be readily machined or honed, and even could be used as wear-in plate before the wear-resistant plate is reached. This type of plate will be illustrated in some of the examples given below.

The method of electroplating the outside of the mandrel instead of the inside of the aluminum alloy housing, also makes possible the use of new wear-resistant plate that is much less expensive than the wearresistant plates in use at the present time, namely thick nickel plate containing about 2.5 wt. of codeposited fine silicon carbide particles, and alternatively thick chromium plate (hard chrome). The new wear-- resistant plate for the Wankel rotary engine is described in Example 1 and consists of electroplated iron containing 2.5 wt. and over of codeposited fine silicon carbide particles. In the low pH (around pH of l very warm (around 160-190F) iron plating baths, it is possible to codeposit on vertical surfaces, twice as much fine silicon carbide particles than with nickel baths. Since less or even no machining of the hard wear-resistant surface need be done, the hardest possible wear-resistant plate can be used, that is, iron or nickel plate with higher percent of codeposited silicon carbide particles. With the plating of the epitrochoidal track on the inside of the aluminum alloy housing, it is too difficult to use a low pH hot iron plating bath because of the attackon the aluminum housing in the areas not receiving plate.

Actually more than one wear-resistant plate can be much more readily used when the mandrel is plated instead of the inside of the aluminum alloy housing as is illustrated in Example 3.

EXAMPLE 1 A steel mandrel with good surface finish and close to the desired dimensions of the epitrochoidal track (approximately 9.5 in. for the longest diameter of the epi trochoidal track, about 7 in. for the pinched-in diameter, and about 2.4 in. for the width is electroplated with the following sequence of plates.

1. A thin lead plate of about 0.1 mil or even less is first plated on the steel mandrel, for example, by using an acid lead fluoborate bath preferably containing one or more addition agents selected from the group consisting of gelatin, naphthol, a nonionic wetting agent, and lignin sulfonate. This can be a l or 2 min. plate.

2. This is followed by a copper plate, preferably from a bright acid copper sulfate plating bath such as the ones described in US. Pat. Nos. 2,707,166, 2,738,318, 3,288,690 and 3,328,273. The copper plate can be about 0.1 to about 1 mil thickness.

3. This is followed by an iron plate containing codeposited fine silicon carbide particles in about 2.5 wt. to about 8 wt. 70. The iron bath can be made up of principally ferrous chloride, about 300 g/ l, with or without about g/ l of calcium chloride, and operated at pH of about I, and bath temperature of about l60-190F The silicon carbide powder concentration can be from about 25 to 150 grams per liter dispersed in the bath by mechanical agitaion. Instead of ferrous chloride, the

bath can be made up of principally ferrous sulfate, operated as described above for the ferrous chloride bath. Lower current densities usually are used for the ferrous sulfate bath, compared to the chloride bath. However, the low pH warm sulfate bath has the advantage that no corrosive hydrochloric acid fumes arise from the bath as is the case with the chloride bath.

The iron plate with the codeposited silicon carbide particles is deposited in a thickness of about 2 mils to even mils or more if desired. The last 1 mil or 2 mils of the thicker plate can use coarser silicon carbide particles in a separate bath to yield a rough plate of firmly imbedded silicon carbide particles in the iron plate. This rough plate will help the bond to the subsequent aluminum alloy housing that is die cast onto this plate. Instead of die casting the aluminum alloy directly onto this iron surface, a thin zinc plate of about 0.1 mil to about 1 mil is plated onto the iron surface and then the aluminum alloy die casting step follows. The zinc plate can be deposited on a smooth iron plate as well as a rough one, and will aid in the bonding of the aluminum alloy to the iron plate. The zinc will melt as the hot aluminum alloy metal hits the zinc and most of the zinc will go into the aluminum alloy and some into the iron plate. This method allows the aluminum alloy to contact iron plate that has no oxide coating on it, the iron surface having been protected by the zinc plate.

The heat from the molten aluminum alloy will cause the melting of the thin lead deposit against the steel mandrel, and this makes possible the removal from the mandrel of the plate with the aluminum housingcattached. The mandrel may be a hollow, that is in the form of an epitrochoidal ring, or have holes in it for cooling purposes. The cooling of the interior of the mandrel (with cold water) will cause contraction and aid in the removal of the plate with its attached housmg.

The final zinc plate is preferably of high purity to help attain the best bonding of the aluminum to the final plate or plates. On the other hand, for the first plate or plates against the mandrel, it is best to use baths with inclusions, as from addition agents to aid in obtaining the poor adhesion necessary for separation from the mandrel. Lead, with its low melting point (327C), loses its adhesion to steel and also the molten lead at these temperatures is practically insoluble in the copper that was plated on top of it. With the cooling of the mandrel, as with the circulation of water through a cylindrical hollow or an epitrochoidal hollow or through holes in the interior of the mandrel, the contraction that results also aids in the separation from the mandrel at the lead interface.

EXAMPLE 2 The same procedure as in Example 1, except that nickel plate with codeposited fine silicon carbide particles to the extent of about 2.5 wt. 7c and higher is used instead of all of the iron plate with codeposited silicon carbide particles or instead of a portion of the iron plate.

The nickel plate with the codeposited silicon carbide particles can be deposited from the acidic baths of the Watts type, high chloride or high bromide types, or sulfamate or fluoborate. Since the acidic fluoborate type will attack titanium anode baskets, the fluoborate type is not preferred over the other types. The nickel baths may contain Class 1 addition agents such as benzene or toluene sulfonamide, di-benzene or di-toluene sulfonimides, o-benzoyl sulfonimide (saccharin), benzene monoor disulfonic acids, benzene and toluene sulfinic acids, naphthalene sulfonic acids, allyl sulfonic acid, o-sulfobenzaldehyde (or the sodium or potassium and similar salts of these various sulfonic acids), sulfonimides, and sulfinic acids or other similar Class I compounds. These Class I compounds cause the incorporation of about 0.01 to about 0.2% sulfur as sulfide into the nickel plate which greatly increases the hardness and tensile strength of the nickel deposit. The pre ferred Class I addition agents for the purpose of this invention and involving the codeposition of silicon carbide fine particles or other fine hard particles are the benzene or toluene sulfonamides, the dibenzene or ditoluene sulfonimides. These Class I compounds allow the maximum amount of codeposition of the fine particles such as silicon carbide for a given concentration of dispersed fine particles in the nickel baths. The preferred concentration of silicon carbide particles is in the range of 25 to grams/liter. The preferred method for the dispersion of the particles in the nickel baths is with air agitation. Wetting agents such as sodium n-octyl sulfate or sodium 2-ethyl hexyl sulfate may be present in the nickel baths. They are normally used at concentrations of about 0.1 to about 1 g/l.

In this method of plating the mandrel, the nickel plate with the codeposited silicon carbide particles can be applied in two or more steps. For example, a thin plain nickel plate of about 0.5 to about 1 mil can be first applied against the copper plate or against the lead plate, then the nickel plate with the codeposited particles of silicon carbide is deposited. With the method of this invention of plating the outside of the mandrel instead of the inside of the aluminum alloy housing, a coarser grade of silicon carbide can be used instead of smooth, very fine silicon carbide particles which must be used when the inside of the aluminum housing is plated, or otherwise as plating proceeds, the plate becomes impossibly rough. In the method of this invention, the first part of the plate is the most important and is smooth because the plating is started against a smooth surface, and with continued plating the roughness that occurs with the larger silicon carbide particles is beneficial for bonding with the cast aluminum alloy. The nickel plate can be deposited with fine smooth micronized silicon carbide particles for the first part of the nickel plate directly on the lead plate or after a plain thin nickel plate has been deposited on the lead plate or on a thin copper deposit plated on the lead plate. Then the nickel plate with codeposited silicon carbide particles can be continued from a nickel bath containing dispersed coarser silicon carbide particles to obtain a rough plate. The latter, with or without a zinc plate, will make possible better adhesion to the aluminum alloy that is die cast against it. The use of a thin, pure zinc plate of about 0.1 mil is preferred for the best bonding procedure whether the rough nickel plate is used or not. The plain nickel plate that may be used at the start of the nickel plating part of the sequence of plates may be made with air agitation turned off in the bath containing the silicon carbide particles, or in a separate bath with no particles, or relatively few parti cles, present.

EXAMPLE 3 The same procedure as in Example 1, except that after the copper plate of step (2), chromium plate is deposited for step (3) instead of the iron plate with codeposited silicon carbide particles. The chromium could be plated directly on the copper plate or on a relatively thin nickel or iron plate. The chromium can be plated in a thickness of about 2 mils to at least about mils. Furthermore, by plating the outside of the mandrel, the highest speed acidic hexavalent chromium plating baths can be used that employ both sulfate and fluoride (or complex fluoride) catalysts such as those described in US. Pat. Nos. 3,334,003, 2,787,588, and 2,640,022. When the inside of the aluminum housing is chromium plated with a thick deposit, it is generally preferred to use the standard hard chrome plating bath employing only the sulfate anion as catalyst because of the attack by the fluoride containing anion on the aluminum in the areas not receiving the chromium plate. In any case, a choice of these chromium plating baths may be made when the outside of the mandrel is plated.

After the chromium plate is deposited, a nickel plate as from a plain Watts bath or a high chloride bath can be deposited on the chromium, after first giving the chromium plate a low pH nickel strike (Woods nickel) to secure bond to the chromium plate. The nickel plate may be as thick as 10 mils if the chromium plate is around 2 to about 6 mils; or about 1 to 5 mils if the chromium plate is about 4 to about 10 mils. The nickel bath can also contain Class I addition agents enumerated in Example 2 as well as codeposited silicon carbide particles. After the nickel plate, a thin pure zinc plate is preferably applied before the aluminum alloy die casting step to secure maximum adhesion. This is especially important if the nickel plate is not made rough as described in Example 2. Instead of nickel plate applied to the chromium plate, iron plate with or without codeposited silicon carbide particles can be applied, as described above. The iron plate is then also preferably given a thin, pure zinc plate before the aluminum die casting step. Actually, even though it takes one more step, it is also possible to plate the nickel or iron plate with about 0.5 to 1 mil of copper plate and then plate the latter with a thin pure zinc plate before the aluminum alloy die casting step. This is also true for Examples 1 and 2 as well. In the aluminum alloy die casting step, the pure zinc with its excellent bond to either nickel, iron, or copper plate will not only preserve the cleanliness of the surface of these latter plates and freedom from oxide, but with the heat from the molten aluminum, the zinc will alloy with these underneath metals, and especially easily with the underneath copper, forming brass, and also will alloy compatibly with the molten aluminum die cast alloy.

EXAMPLE 4 The surface of the steel mandrel is first prepared as follows. The steel mandrel is adherently plated with nickel. This can be done from a plain Watts nickel bath or other acidic nickel baths such as high chloride, or sulfamate, or from a semi-bright or bright nickel plating bath. A bright, smooth nickel plate is preferred. Duplex nickel plate may be used for maximum corrosion protection of the mandrel. The nickel plate is then chromium plated with about 0.01 to about 0.l mil of chromium. The mandrel may be directly chromium plated but it is preferred to nickel plate it first, and especially with bright nickel as the final plate before the chromium plate. In this way, the mandrel will not rust when not in use. Also, it may be desirable to use a copper undercoat to the nickel or to the chromium to help for heat or cold transfer. The mandrel with its chromium plate is now the finished mandrel ready for continuous use for the plate sequences and the final aluminum alloy die casting steps as described below.

The chromium plated surface of the mandrel is first plated with about 0.1 mil or even less of zinc from an acidic zinc plating bath. Zinc from acidic baths, especially at pH values around 2, adheres well to a chromium plate. The zinc plate is then overlaid with copper from an alkaline bath, such as from a cyanide copper bath (about 2 to 6 min. plating). Further copper from the alkaline bath, but preferably from a bright acid cop per plating bath, can now be deposited to a thickness of about 0.5 to about 1 mil. Then this is followed by step (3) and subsequent steps of Example 1, or the steps in Example 2 equivalent to :step (3) and subsequent steps, or the equivalent steps in Example 3. During the final aluminum alloy die casting step, or after a temperature of about 350 to 500C is reached at the zinc interface, (the m.p. of zinc is 420C), the zinc alloys with the copper overlay plate to form brass and leaves the chromium plated surface of the mandrel bright and practically untouched. The chromium surface of the mandrel can be reused with simple cleaning and cathodic activation, or a fresh thin flash of chromium can be applied to the mandrel before starting the thin zinc plate step.

EXAMPLE 5 Instead of a thin lead plate of about 0.l mil (it can be somewhat thinner or thicker), a thin cadmium plate of about 0.1 mil can be plated directly on the steel mandrel, to serve as the means of detachment or parting from the mandrel after the wear-resistant plate or plates and other plates are applied and after the step of die casting the aluminum alloy housing onto the final plate. Cadmium melts at 321C, very close to the melting point of lead. The cadmium plate is overplated with copper from an alkaline bath, or it may be overplated with zinc and then with copper, or it may be overplated with nickel. The overplates may be about 0.1 to about 1 mil. The heat from the molten aluminum die casting step will cause the bond to break in the cadmium plate which melts and also tends to diffuse into an overlay plate of copper or zinc. While the thin lead and cadmium plates make possible good separation from the mandrel after the aluminum alloy die casting step, it would be preferred, in general, to use zinc for the parting step as described in Example 4. The zinc is preferred because it is far less toxic than lead and cadmium, and far less expensive than cadmium. Thin tin plate (mp. 232C) could also be used with or instead of lead and cadmium, but it is much more expensive and less plentiful than zinc.

EXAMPLE 6 The same steps as in Examples 1 to 4 inclusive, except that after the wear-resistant plates, that is, iron or nickel with codeposited silicon carbide particles, or chromium, have been deposited, the next plate to be deposited is nickel, iron or copper in thicknesses of about 0.5 to about 2 mils, plated from baths containing from about 25 to grams/liter of Saran or PVC or plystyrene dispersed in these baths (see US. Pat. Nos. 3,676,308, 3,356,467). The chromium plate has to have a nickel, iron or cobalt strike plate from a low pH bath before the application of the copper, nickel or iron plate containing the densely codeposited plastic particles. With Saran especially, it is possible to codeposit comparatively large particles on vertical cathodes, for example, particles around 2 mils in diameter. After one of these composite plates is deposited, the plastic particles may be dissolved out with solvents, thus leaving a plate full of holes. This plate can be plated with a thin pure zinc plate and the aluminum alloy casting step is then made. The holes in the plate will help to obtain maximum adhesion of the aluminum alloy to the plated metals. Instead of dissolving out the plastic particles from the plate, another plate, preferably iron or nickel without codeposited plastic particles, is deposited in a thickness of about 0.5 to 2 mils on the surface of the plate containing the densely codeposited plastic insulating particles. This latter plate will then be highly porous and will greatly help in obtaining the best bonding with the aluminum alloy die casting. Instead of codepositing plastic particles, excellent results for achieving a porous final plate without the need of dissolving out the particles, is to use barium or strontium sulfate particles of diameters of about 5 microns or less dispersed in the baths in a concentration of about 25 to 150 grams per liter in the nickel, iron or copper baths (see US. Pat. Nos. 3,672,970, 3,666,636, 3,152,973) and then depositing plate of plain nickel or iron on top of the plate containing the densely codeposited inor ganic particles. Barium sulfate particles agglomerate and are codeposited as agglomerates, that is, they are present in the plate as agglomerates of about 25 micron l mil) size, even though the ultimate particle size may average about 0.2 to 2 microns size.

EXAMPLE 7 The same steps as in the previous examples except that the "break-in plates that may be used or machined to size of nickel or iron preceding the wearresistant plates, are deposited with inclusions of lubrieating particles such as those of graphite, molybdenum sulfide, mica, strontium sulfate and barium sulfate. Barium sulfate is especially excellent in this respect, and can be densely codeposited when dispersed in copper, nickel or iron baths in concentrations of about 25 to 150 grams per liter as mentioned in Example 6. Graphite and molybdenum sulfide particles can be densely codeposited even when dispersed in the baths in concentrations as low as 1 gram per liter. These particles tend to cause rough plate when thicker than about 0.5 mil plate is deposited.

EXAMPLE 8 Another method of obtaining good bond of the aluminum alloy die casting to the final plate of nickel, cobalt, iron or their alloys is to electroplate an aluminum film on to these metals prior to the die casting step. One of the best baths to plate out an excellent aluminum deposit is from the organic baths or their modifications developed at the US. Bureau of Standards (see US. Pat. Nos. 2,651,608; J. Electrochem. Soc., Vol. 99, p. 234 (1952); ibid., Vol. 103, p. 653 and p. 657 (1956).

An adherent aluminum film can also be obtained by dipping the final plate of nickel, cobalt, or iron or their alloys into a flux and a molten bath of aluminum or aluminum alloy preferably containing about 5 to about 12% of silicon. The silicon in these concentrations and up to the eutectic point lowers the melting point of the aluminum. This aluminum film will greatly aid in obtaining a strong bond of the aluminum alloy die casting to the final plate. The latter hot dip bath would best be used when zinc plate is used as the parting plate from the mandrel.

EXAMPLE 9 In the Examples l-8, the method of securing the parting of the electroformed plate from the mandrel depends either on the melting of a thin plated metal with a lower melting point than aluminum, for example lead plate, tin, cadmium, zinc, antimony, bismuth and alloys of these metals, or the diffusion of one metal into another as cadmium into copper, tin into copper, zinc into copper, or both on melting and diffusion, as with cadmium, tin or zinc against copper, and lead against zinc or tin.

Another way to obtain parting is to plate the steel mandrel with a poorly adherent plate, but one which will not blister. For example zinc plating a chromium plated mandrel, using a slightly acidic zinc sulfate bath of pH of about 3 to 5 yields a zinc plate on the chromium that has poor bond. Silver plating the steel mandrel with a cyanide silver plating bath yields a silver plate with poor bond, especially ifa regular silver cyanide bath is used without a preliminary low metal, high cyanide strike. Also, if the steel mandrel is plated with copper and then the copper plate is treated by dipping in a 1 g/l solution of selenious oxide or sodium selenite, a subsequent plate of copper from an acid bath will have poor adhesion. If the mandrel is nickel plated with good bond, the nickel plate can then be passivated by using anodic alkaline cleaning with no acid dips, and a second nickel plate, preferably from a bath of pH of 4 to 5, will have poor adherence to the first nickel plate. Or if the first nickel plate after cathodic cleaning or just after a fresh nickel strike, is dipped in about 3 g/l aqueous solution of sodium or potassium dichromate or chromic acid and then thoroughly rinsed, the subsequent nickel plate will be poorly adherent. All of these methods depending on poor adhesion without melting or diffusion, will require for best parting results, a cooling of the mandrel to obtain contraction. This step together with the expansion effect from the heat from aluminum alloy die casting step will greatly aid in the parting step.

EXAMPLE 10 Another very good method to secure good parting is to electrodeposit a porous film of barium sulfate, strontium sulfate, mica, graphite, molybdenum sulfide or other similar lubricating particles on to the mandrel. This can be accomplished by dispersing about 25 to about 150 grams per liter of one or more of these powders in an aqueous solution of sodium or potassium sulfate or other non-plating aqueous electrolyte, for example, in an electrolyte of 150 grams per liter of sodium sulfate, and using cathode current densities of about 40 to amps./sq.ft. and depositing the porous film in about 5 to 10 minutes (see US. Pat. No. 3,687,824). Then on to the mandrel on which this porous film has been deposited, is plated thin cadmium, tin, lead or zinc or their alloy plates and the parting will be greatly facilitated with the presence of these lubricating particles, especially barium sulfate particles.

In general, for the parting plate or plates from the mandrel, that is, lead, cadmium, tin or zinc or their alloys, it is best to use addition agents in the baths, since the inclusions from the addition agents tend to cause poor adhesion when the plates become hot from the aluminum alloy die casting step or from any molten aluminum dips. When zinc plate is used to aid in obtaining good bond of the aluminum alloy die casting to the final plate or plates, then it is generally preferred to use no organic addition agents, or a minimum of addition agents. The plate with inclusions may cause a dark film or stain on the mandrel or the chromium plated mandrel, but this is no problem and can be readily cleaned. Also, the final relatively thin plates of nickel, cobalt or iron or their alloys are preferably plated with no organic addition agents or a minimum of addition agents present in the bath. This helps to obtain the best bond with the aluminum alloy die casting. These final plates or plate can be very thin plates on top of the same plates deposited from baths containing organic addition agents such as Class 1 addition agents in their usual concentrations.

The nickel, cobalt, iron or their alloy plating baths are all high speed plating baths and can be used at their highest current densities without problems of polarized anodes as occurs when the track on the inside of the aluminum housing is plated and inside anodes of necessarily limited anode area must be used. This is contrary to the case when the outside of the mandrel is plated, and the anode area can be many times the cathode area, and thus the highest current densities may be used without appreciable polarization of the anodes.

In U.S. Pat. No. 3,514,389, May 26, 1970 there is described a process and apparatus for the plating of the inner surface of trochoidal engine housings with a nickel plate containing codeposited fine hard particles in the matrix of the nickel plate to obtain improved wearresistance of highly stressed surfaces. See also US. Pat. No. 3,152,971, Oct. 13, 1964, wherein a nickel plating bath containing fine dispersed hard particles such as silicon carbide, aluminum oxide, boron carbide, titanium carbide are described.

The mandrels for each electroformed rotor track can be racked for the electroplating steps in stacks of about 5 or units or more. The insulating separators between each mandrel may be plastic or rubber or other insulating material not harmful to the plating baths. The electrical contacts between the stacked mandrels can be made with rods through the mandrels which can also serve for alignment and for rack holders. After the mandrels are plated, they are disassembled for the step of die casting the aluminum alloy onto the final coating. The final coating may be given an activating cleaning before the aluminum alloy die casting step.

In Example 1, the advantage of having no acid fumes arising from the warm low pH (about 0.6 to 1.5) ferrous sulfate baths compared to the low pH warm ferrous chloride baths was mentioned. One fault of the ferrous sulfate bath is that the solubility (about 150 grams per liter) of ferrous sulfate at room temperature is much lower than at 150 l80F (solubility about 400 g/ l and when the baths are not in use and are allowed to cool, crystallization of ferrous sulfate may occur. The use of ferrous methane sulfonate along with at least 150 g/ l ferrous sulphate, and the use of methane sulfonic acid to maintain the low pH will help in obtaining maximum solubility at room temperatures without the addition of high concentration of chloride. With about g/l of fine silicon carbide dispersed in the low pH warm ferrous sulfate baths, at least 8 wt. of silicon carbide can be codeposited on vertical surfaces.

The casting of the aluminum alloy housing may be permanent mold casting or die casting. Aluminum as the final plate or film prior to the aluminum casting step would be the best final plate, but it would also be the most inconvenient one, compared to nickel or iron plate with and without a final zinc plate. Molten aluminum directly against copper tends to form a very brittle bond.

The mandrel with the plates on it may be preheated before the aluminum alloy casting step, and also beneficial heat treatments may be made after the completion of the aluminum alloy casting step. a

As mentioned in Example 7, the first deposits other than plates used for separation from the mandrel that may be conveniently used prior to the hard wearresistant plate, may be employed as break-in plate, especially when these first plates are iron, cobalt or nickel with or without codeposited soft lubricating particles such as barium sulfate, mica, etc. Metals like cop per, zinc, brass, etc., are best machined off. The iron, nickel, cobalt or their alloy plate without codeposited hard particles (if such plate is used prior to the wearresistant plate) can of course, be machined down or almost down to the wear-resistant plate and even partly down into the latter plate.

The last plates deposited after the hard wear-resistant plate (hard chromium or iron or nickel with codeposited fine hard particles such as silicon carbide, aluminum oxide, etc.) are preferably iron plate with a final zinc plate. Molten aluminum against iron plate, while forming a brittle alloy, nevertheless yields a strong bond, unlike the case with molten aluminum against copper. It is best to have the iron plate rough as accomplished in Example 2 or accomplished by sand blasting, or by having multitudinous holes in the iron plate as in Example 6, and to have a zinc plate as the final plate against the iron for very good bond with the cast aluminum alloy. The zinc plate may be a galvanized one or an electroplate.

Thus, one of the shortest and most economical of the sequences of plates deposited preferably on a chromium plated mandrel would be; (I) about 0.l mil zinc plate from an acidic zinc electroplating bath, (2) followed by about 0.l mil of copper from an alkaline copper electroplating bath (pyrophosphate, amine, or cyanide type), (3) followed by about 2 to about 6 mils of iron plate containing about 2.5 to about 8 wt. percent of fine codeposited silicon carbide or other similar hard particlas, (4) followed by about 0.3 to about 2 mils of zinc plate, (5) followed by the aluminum alloy casting step.

It should be emphasized that a thicker zinc plate can be used, but in experimental tests, about 0.3 mil zinc and 1 mil zinc plate on the iron plate gave excellent bonding results with the aluminum alloy. The zinc plate also made possible good bonding results with the aluminum alloy when the zinc was plated on nickel with codeposited silicon carbide particles instead of iron plate. The zinc plate makes possible good bonding results with the aluminum alloy when the zinc is deposited on cobalt plate, or on binary and ternary alloys of iron, nickel and cobalt. The zinc may be a high zinc alloy plate, but it is best electroplated as a substantially pure zinc. The above sequence of plating steps can be carried out in less than 2 hours plating time, especially in case operational experience (50,000 mile tests) show that only 2 to about 4 mils of wear-resistant plate is needed. This would be true whether iron or nickel plate with codeposited hard particles is used as the wearresistant plate.

Instead of the thin copper plate (step 2 above) directly on the first zinc plate (step 1 above) used for the parting plane step from the mandrel, one can use thin brass, bronze, tin, cadmium, lead, silver, nickel, cobalt and iron and the binary and ternary alloys of these metals when these metals are plated from neutral or alkaline solutions. Copper or nickel from an alkaline bath is preferred. Nickel from an alkaline bath in thicknesses of about 0.1 to about 1 mil may be used with certain advantages, for example, as a wear-in plate. In this case it can also contain codeposited anti-friction particles such as barium sulfate. The metals like copper, tin, etc., would not be very suitable for wear-in plate because they would be rapidly oxidized into powdery materials during combustion unlike the case with nickel, cobalt, iron or their alloys. The mandrel may be a metal made from iron, steel, brass, copper or other metals of melting point higher than molten aluminum. The mandrel may even have a plastic or polymer surface which can be metallized in order to be electroplated for the sequence of plating steps described above. When the metal mandrel is designed, it is best to have a strongly bonded plate of nickel, cobalt, iron or their alloys with preferably a final plate of chromium integrally bonded to the mandrel as already described.

Copper plate may be used between any combination of nickel, cobalt, and iron plates, or their alloys, or between chromium plate and these metals. The copper plate will help in obtaining a more uniform and faster heat transfer especially when maximum thicknesses of chromium or nickel, iron, or cobalt plates are used.

The electroformed sequences of plates described herein for the formation of the rotor track of the Wankel engine has many advantages over the plating of the inside of the aluminum alloy housing with nickel containing codeposited silicon carbide particles or by using the transplant coating process (TCP) of Doehler-Jarvis described by A.F. Bauer in Soc. of Automotive Engineers SAE Paper 369 C 1961 followed by the aluminum alloy housing casting step, followed by thick chromium plating as described in SAE Paper 700079 presented by K. Yamamoto and T. Kuroda in January 1970 at the Automotive Engineering Congress in Detroit, Michigan. The advantages are (l) the plating is done on the outside of a cathode (the mandrel) instead of the inside of the housing, (2) it makes possible the use of the least expensive metal, iron, for the wear-resistant plate, (3) combinations of wearresistant plate may be more readily used, (4) thinner wearresistant plate can be used and (5) the least final machining needs to be done.

What is claimed is:

1. A method for forming a housing comprised of aluminum for the rotor track of a rotary internal combustion engine, which comprises the steps of:

l. electrodepositing a first layer on to a mandrel of the shape of the track of the rotor, said first layer being separable from the mandrel during subsequent steps;

2. electrodepositing a wear resistant layer on to said first layer;

3. forming on to the wear resistant layer a metallic layer useful for improving adhesion of the aluminum casting to the previously formed wear resistant layer;

4. electrodepositing zinc on to the mandrel, electrodepositing copper on to the zinc deposit, electrodepositing a plate comprised of iron of a thickness of about 1 to about 10 mils containing about 2 to about 8 weight percent fine silicon carbide particles, and electrodepositing a zinc deposit of a thickness of about 0.1 mil to about 2 mils;

5. then casting a housing comprised of aluminum;

and

6. separating the housing from the mandrel.

2. A method according to claim 1, wherein Step 4 is a thin nickel plate instead of the copper.

3. A method according to claim 1, wherein Step 3 isa wear resistant plate comprised of nickel containing about 2 to about 4 weight percent of silicon carbide particles.

4. A method according to claim 1, wherein Step 3 is a wear resistant plate comprised of cobalt containing about 2 to about 4 weight percent of silicon carbide particles.

5. A method according to claim 1, wherein the wear resistant plate of Step 3 is an electrodeposited plate of about 1 to about 4 mils thickness of chromium.

6. A method according to claim 3, further comprising electrodepositing copper on the nickel plate.

7. A method according to claim 5, wherein the chromium plate is followed by electrodepositing a plate of about 2 to about 10 mils in thickness comprised of nickel.

8. A method according to claim 5, wherein the chromium plate is followed by electrodepositing a plate of about 1 to about 6 mils in thickness comprised of cobalt.

9. A method according to claim 5, wherein the chromium plate is followed by electrodepositing a plate of about 2 to about 10 mils thickness comprised of iron. l 

1. A METHOD FOR FORMING A HOUSING COMPRISED OF ALUMINUM FOR THE ROTOR TRACK OF A ROTARY INTERNAL COMBUSTION ENGINE, WHICH COMPRISES THE STEPS OF:
 1. ELECTRODEPOSITING A FIRST LAYER ON TO A MANDREL OF THE SHAPE OF THE TRACK OF THE ROTOR, SAID FIRST BEING SEPARABLE FROM THE MANDREL DURING SUBSEQUENT STEPS;
 2. ELECTRODEPOSITING A WEAR RESISTANT LAYER ON TO SAID FIRST LAYER;
 2. A method according to claim 1, wherein Step 4 is a thin nickel plate instead of the copper.
 3. A method according to claim 1, wherein Step 3 is a wear resistant plate comprised of nickel containing about 2 to about 4 weight percent of silicon carbide particles.
 3. forming on to the wear resistant layer a metallic layer useful for improving adhesion of the aluminum casting to the previously formed wear resistant layer;
 3. FORMING ON TO THE WEAR RESISTANT LAYER A METALLIC LAYER USEFUL FOR IMPROVING ADHESION OF THE ALUMINUM CASTING TO THE PREVIOUSLY FORMED WEAR RESISTANT LAYER;
 4. ELECTRODEPOSITING ZINC ON TO THE MANDREL, ELECTRODEPOSITING COPPER ON TO THE ZINC DEPOSIT, ELECTRODEPOSITING A PLATE COMPRISED OF IRON OF A THICKNESS OF ABOUT 1 TO ABOUT 10 MILS CONTAINING ABOUT 2 TO ABOUT 8 WEIGHT PERCENT FINE SILICON CARBIDE PARTICLES, AND ELECTRODEPOSITION A ZINC DEPOSIT OF A THICKNESS OF ABOUT 0.1 MIL TO ABOUT 2 MILS;
 4. electrodepositing zinc on to the mandrel, electrodepositing copper on to the zinc deposit, electrodepositing a plate comprised of iron of a thickness of about 1 to about 10 mils containing about 2 to about 8 weight percent fine silicon carbide particles, and electrodepositing a zinc deposit of a thickness of about 0.1 mil to about 2 mils;
 4. A method according to claim 1, wherein Step 3 is a wear resistant plate comprised of cobalt containing about 2 to about 4 weight percent of silicon carbide particles.
 5. A method according to claim 1, wherein the wear resistant plate of Step 3 is an electrodeposited plate of about 1 to about 4 mils thickness of chromium.
 5. THEN CASTING A HOUSING COMPRISED OF ALUMINUM; AND
 5. then casting a housing comprised of aluminum; and
 6. SEPARATING THE HOUSING FROM THE MANDREL.
 6. separating the housing from the mandrel.
 6. A method according to claim 3, further comprising electrodepositing copper on the nickel plate.
 7. A method according to claim 5, wherein the chromium plate is followed by electrodepositing a plate of about 2 to about 10 mils in thickness comprised of nickel.
 8. A method according to claim 5, wherein the chromium plate is followed by electrodepositing a plate of about 1 to about 6 mils in thickness comprised of cobalt.
 9. A method according to claim 5, wherein the chromium plate is followed by electrodepositing a plate of about 2 to about 10 mils thickness comprised of iron. 